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For every blush that’s housed in its original glossy packaging or for every eyeshadow that gets a velvet pouch to keep it safe and scratch-free, there’s a dirty makeup brush somewhere nearby that’s being neglected. Maybe you give them the shaft because it’s really annoying to clean them, but they’re actually one of the most vital beauty tools to maintain. If you don’t, they end up holding on to a lot of really, really gross stuff.

‘Skin oils can build up over several months’ time and if make-up brushes are kept in the bathroom close to the toilet they can harbor some nasty bacteria,’ says Dr. Elizabeth Tanzi, a D.C.-based dermatologist and the founder of Capital Laser and Skin Care.

You might want to give them a wash immediately after reading about these potential consequences.

1. Acne

By not washing your makeup brushes, you’re basically putting yourself in a time machine that goes all the way back to puberty.

‘Pores undergo constant sebum production and cell turnover,’ says Dr. Tanzi. ‘If anything gets in the way of that process, such as bacteria, outside debris (make-up), and excessive oil buildup on the brushes, then blackheads and whiteheads can develop.’ Essentially, if you wash your face to help avoid acne only to use a dirty brush to apply make-up afterwards, you may as well have skipped that healthy-skin step because you’ve just plugged your pores with oil, bacteria, and old make-up. Gross.

2. Viruses

‘Viruses can live for long periods of time on make-up brushes,” says Dr. Tanzi, specifically pointing out the common cold virus and the cold sore virus. (This doesn’t just apply to sharing brushes with other people!)

There’s a common school of thought that you can never get the same cold twice because your body has built up antibodies to it, but because viruses can live for so long on your brush, they have time to mutate into a brand-new virus that your body can’t recognise and fight off.

3. Other nasty infections

Bacteria too can live for an incredibly long time on brushes, according to Dr. Tanzi, leading to staph infections if it comes into contact with an open wound and pink eye (or conjunctivitis), which can be easily spread through any kind of eye make-up.

4. Irritation

‘The issue here is when the brushes are so dirty and oily that they deposit this debris on the surface of the skin,’ says Dr. Tanzi. ‘Debris on dirty make-up brushes that builds up over months (or even years in some cases) can cause sensitive skin to become irritated, which may present as redness and bumps.” The debris – in this case oil, makeup and bacteria from your brush – sits on top of the skin and prevents it from a natural cell turnover process, keeping it from being able to properly regulate itself according to the environment around it. So, for example, in dry weather, skin will start to flake (not cute), and when it’s hot, skin goes into oil overdrive (so you look like a greaseball).

5. Dullness

‘Sebum is the waxy or oily material produced by the skin that comes out through the pores. If someone is acne-prone, they are likely producing an excess of sebum. When that sebum builds up on a brush, it just gets applied right back in to the skin,’ says Dr. Tanzi. ‘Too much sebum on the surface of the skin reduces the reflection of light off the skin, which can make it look dull.’ In that case, it practically negates why you were reaching for your make-up in the first place.

6. Ruined tools

It’s not a skin concern, but it should make your wallet wary. Brushes, whether they’re made of natural or synthetic bristles, need to be cleaned regularly to last. When they’re clogged with gunk and spread across your face, bristles can bend or fall out. To maintain the integrity of a brush, it should be given a deep clean once per week. That means carefully washing them with a gentle cleansing agent (baby shampoo works really well), rinsing them thoroughly, and letting them air dry. Otherwise, you’ll keep needing to replace them, which over time could knock your bank balance a bit – and wouldn’t you rather spend that on new, cool make-up instead?